1. Field of the Invention
The present invention relates to a positioning apparatus, and more specifically, it relates to a positioning apparatus used for positioning a substrate in a photolithography machine.
2. Description of the Related Art
A semiconductor photolithography machine makes exposure light incident on an original pattern drawn on a reticle. The light transmitted or reflected by the reticle is reduced with an exposure optical system, and the reduced pattern is projected onto a semiconductor substrate (wafer). In this way, a semiconductor photolithography machine performs exposure operation. The reticle having the pattern to be transferred is mounted on a reticle stage and positioned at a predetermined position. The reticle is irradiated from above with exposure light by an illumination system. The exposure light then enters a reduced projection optical system. This optical system forms an image at a predetermined position. A wafer stage carries and positions a wafer such that a predetermined area on the wafer is positioned at the point where the image is formed. The positional information of the wafer relative to the wafer stage has been obtained in advance by measuring the position of an alignment mark on the wafer with an alignment optical system. When exposure is performed, the wafer is positioned at the predetermined position on the basis of this alignment information.
Throughput is one of the indicators of the performance of a photolithography machine. The throughput is expressed as the number of wafers that the photolithography machine can process per unit time. In order to increase the throughput, it is necessary to move the wafer stage in a short time. For this purpose, it is necessary to increase the moving velocity in addition to the acceleration and deceleration when the wafer stage is moved. In order to achieve high acceleration and deceleration, high moving velocity, and highly accurate positioning performance, conventional wafer stages generally have a coarse/fine-motion multistep configuration including a fine-motion stage and a coarse-motion stage. The fine-motion stage carries and positions a wafer with high accuracy. The coarse-motion stage moves the fine-motion stage in the horizontal direction at high acceleration and deceleration and high moving velocity. In this configuration, a coarse-motion actuator needs to accelerate and decelerate the combined mass of the coarse-motion stage and the fine-motion stage. The higher the acceleration, the greater the necessary thrust. Consequently, the coarse-motion actuator tends to be large, and the entire stage apparatus also tends to be large. This tendency is undesirable because it causes an increase in the production cost and an increase in the area for installing the apparatus.
In addition, recently, a twin-stage configuration has been proposed. In the twin-stage configuration, while a wafer on one stage is exposed, another wafer to be exposed next is mounted on the other stage and aligned. In the twin-stage configuration, two stages individually convey wafers, and each stage repeats a cycle of wafer mounting, alignment operation, exposure operation, and wafer pickup. Therefore, the two stages use a common alignment optical system, exposure optical system, and wafer exchanger at different times. In the case of the conventional coarse/fine-motion stage, a complex configuration is necessary to interchange positions of two stages.
To solve this problem, a surface-motor stage has been devised. FIGS. 15A and 15B show a surface-motor stage apparatus that can perform positioning in six directions by Lorentz force (see Japanese Patent Laid-Open No. 2004-254489, corresponding to US Patent Application No. 2004-126907).
The stage apparatus includes a stage (mover) 110 and a coil unit (stator) 100. The stage 110 has a magnet unit 114 on the underside. The coil unit 100 faces the magnet unit 114. The magnet unit 114 includes a plurality of permanent magnets. The plurality of permanent magnets are arranged in the XY direction in a so-called Halbach array. The coil unit 100 includes a plurality of coils. The coil unit 100 includes a layer 116a of coils arranged in the X direction and a layer 116b of coils arranged in the Y direction. Although not shown in FIG. 15A, the coil unit 100 further includes another layer of coils arranged in the X direction and another layer of coils arranged in the Y direction. By selectively applying a current to these coil layers, a Lorentz force is generated between the magnet unit 114 and the coil unit 100, and consequently the stage 110 can be moved.
Using the coil layer 116a, a thrust in the X direction is given to the stage 110. Using the coil layer 116b, a thrust in the Y direction is given to the stage 110. Using the other coil layers, thrusts in the Z direction (vertical direction), θx direction (rotating direction around the X axis), θy direction (rotating direction around the Y axis), and θz direction (rotating direction around the Z axis) are given to the stage 110. The weight of the stage 110 is supported by the coil layers that give the stage 110 the thrust in the Z direction.
The coils constituting each coil layer generate desired forces in pairs. Each pair of coils (a phase A coil and a phase B coil) is adjacent to each other. The magnet unit 114 has cyclic (for example, sine-wave) magnetic-flux-density distributions in the X direction and the Y direction. Therefore, when a certain current is applied to the phase A coils, the generated thrust is a sine wave whose argument is a position of the magnet unit 114 relative to the coil unit 100. The magnet unit 114 and the coil unit 100 are arranged such that, when a certain current is applied to the phase B coils, the generated thrust is a sine wave that is out of phase with the thrust of the phase A coils by 90 degrees. Therefore, by obtaining a rectification value from the position of the magnet unit 114 relative to the coil unit 100 and applying a current multiplied by the rectification value to the phase A coils and the phase B coils, desired forces can be generated.
However, in the case where the weight of the stage is supported using coils arranged in the X direction (or the Y direction), the application points of the forces applied to the stage to support the weight of the stage change as the stage moves in the X direction (or the Y direction). That is to say, when the stage is at a position, only the phase A coils apply forces to the stage; when the stage is at another position, only the phase B coils apply forces to the stage; and when the stage is at yet another position, both phase A coils and phase B coils apply forces to the stage. Such change in the application points of forces can cause undesirable deformation of the stage.
In photolithography machines, in general, a laser interferometer is used for measuring the position of a stage. A reflecting surface (mirror) is provided in the stage. The laser interferometer measures the position of the reflecting surface (mirror) by irradiating the reflecting surface (mirror) with laser light. Therefore, the positional relationship between the reflecting surface (mirror) and the exposed area on the wafer must be fixed. If the above-described deformation occurs, the positional relationship between the reflecting surface (mirror) and the exposure area on the wafer changes and therefore the exposure accuracy deteriorates.